This invention relates to a hydraulic circuit for a working machine such as a hydraulic excavator, and more particularly to a hydraulic circuit for a working machine suitable for use with a working machine based on a hydraulic excavator and having a multistage expansion arm for caisson type excavation.
FIG. 8 is a schematic side elevational view showing a hydraulic excavator (working machine) to which a common multistage expansion arm is attached. The hydraulic excavator includes a lower traveling unit 1, an upper revolving unit 2 coupled for revolution to the lower traveling unit 1, a boom 3 mounted for swinging motion on the upper revolving unit 2, a multistage expansion arm (expansible arm) 4 mounted for swinging motion at a tip end of the boom 3 and having an expansion/contraction function, a clamshell bucket 5 mounted at a tip end of the multistage expansion arm 4, and so forth.
A boom cylinder 3a is provided between the boom 3 and the upper revolving unit 2, and the boom 3 is driven to swing in response to an expansion/contraction movement of the boom cylinder 3a. Similarly, an arm cylinder 4a is provided between the boom 3 and the multistage expansion arm 4, and the multistage expansion arm 4 is driven to swing in response to an expansion/contraction movement of the arm cylinder 4a. It is to be noted that a cylinder 11 [refer to FIG. 9] is provided for the multistage expansion arm 4 and can expand and contract the multistage expansion arm 4.
The clamshell bucket 5 is configured for opening and closing movement by causing a hydraulic cylinder 5a [refer to FIG. 9] provided in the inside thereof to operate.
FIG. 9 is a schematic view showing a general configuration of a hydraulic circuit for the hydraulic excavator described above. It is to be noted that a pilot circuit is not shown in FIG. 9. Referring to FIG. 9, reference numeral 6 denotes a prime mover, reference characters 7a, 7b denote each a hydraulic pump (pressure source) driven by the prime mover 6, and reference numeral 8 denotes a control valve unit for controlling pressure oil (operating oil) from the hydraulic pumps 7a, 7b to distribute the flow rates of the pressure oil to various actuators which are hereinafter described. Reference numeral 9 denotes a revolving motor for driving the revolving motor 9, and reference characters 10a, 10b denote each a traveling motor for driving a traveling apparatus not shown provided on the lower traveling unit 1.
Reference character boom cylinder 3a denotes a boom cylinder, 4a an arm cylinder, 5a a bucket cylinder for opening and closing the clamshell bucket, 11 a telescopic cylinder for expanding and contracting the multistage expansion arm 4, 12 a slow return valve provided in a rod side chamber 11b of the telescopic cylinder 11, and 17 a tank.
When operating oil is supplied into a hydraulic chamber at an upper portion in the figure of the bucket cylinder 5a to move the bucket cylinder 5a downwardly in the figure, the clamshell bucket 5 is opened. A restrictor (orifice) is formed in the inside of the slow return valve 12 and prevents sudden expansion of the multistage expansion arm 4 by its own weight.
Reference numeral 13 denotes a telescopic control valve for expanding or contracting the telescopic cylinder 11 built in the control valve unit 8, 14 a bucket control valve for operating the bucket cylinder 5a, reference characters 15a, 15b denote each a telescopic remote control valve for controlling the telescopic control valve 13, reference numeral 15 denotes a telescopic remote control lever for controlling movement of the telescopic remote control valves 15a, 15b, valves 16a, 16b are bucket remote control valves for controlling the bucket control valve 14, and reference numeral 16 denotes a bucket remote control valve for controlling operation of the bucket remote control valves 16a, 16b. 
Of the components given above, the telescopic remote control valve 15a is a remote control valve (opening operator) for expanding the telescopic cylinder 11, and when the telescopic remote control lever 15 is tilted rightwardly in the figure, the telescopic remote control valve 15a is opened and a pilot pressure corresponding to the operation amount of the telescopic remote control lever 15 is outputted.
The bucket remote control valve 16a is a remote control valve (opening operator) for causing the clamshell bucket 5 to perform an opening movement, and when the bucket remote control lever 16 is tilted rightwardly in the figure, the bucket remote control valve 16a is opened and a pilot pressure corresponding to the operation amount of the bucket remote control lever 16 is outputted.
Reference characters 103a, 104a, 109, 110a, 110b denote control valves for controlling movement of the boom cylinder 3a, arm cylinder 4a, revolving motor 9, and traveling motors 10a, 10b, respectively, and reference numeral 120 denotes a traveling straightforward valve for keeping straightforward traveling of the hydraulic excavator. It is to be noted that detailed description of the valves just mentioned is omitted.
Referring to FIG. 9, if the telescopic remote control lever 15 is operated to open the telescopic remote control valve 15a, then a pilot pressure acts upon a pilot port 13a to change over the telescopic control valve 13 of the control valve unit 8 from a chamber N to another chamber X. Then, pressure oil is supplied from the hydraulic pumps 7a, 7b into a head side chamber 11a of the telescopic cylinder 11 while pressure oil in the rod side chamber 11b is introduced into the tank 17 through the slow return valve 12 and the telescopic control valve 13.
At this time, since the weights of the multistage expansion arm 4 and the clamshell bucket 5 themselves act in the rod side chamber 11b of the telescopic cylinder 11, a high pressure is generated in the rod side chamber 11b, but the pressure in the head side chamber 11a becomes low since no load is applied to the head side chamber 11a. 
Accordingly, if the bucket remote control lever 16 is operated in order to open the clamshell bucket 5 while the multistage expansion arm 4 is being extended, then most of pressure oil in the hydraulic pumps 7a, 7b flows into the head side chamber 11a of the telescopic cylinder 11 which has a lower working pressure. Consequently, sufficient pressure oil is not supplied into the bucket cylinder 5a and the speed at which the clamshell bucket 5 is opened is reduced, resulting in a subject that the operability is deteriorated.
The present invention has been made in view of such a subject as just described, and it is an object of the present invention to provide a hydraulic circuit for a working machine which prevents drop of the working speed of a clamshell bucket upon expansion of an expansion arm so that improvement in operability is achieved.
In order to attain the object described above, according to an aspect of the present invention, a hydraulic circuit for a working machine which includes an expansion arm and a clamshell bucket attached to a tip end of the expansion arm and is configured such that the expansion arm and the clamshell bucket are operated by pressure oil supplied from a common pressure source is characterized in that it comprises pressure reduction means for reducing an operating pressure for driving the expansion arm to the expansion side based on an operating pressure for opening the clamshell bucket.
Accordingly, with the hydraulic circuit for a working machine of the present invention, when an operation for opening the clamshell bucket is performed while the expansion arm is being expanded by reducing the operating pressure for driving the expansion arm to the expansion side based on the operating pressure for opening the clamshell bucket, supply of pressure oil for driving the expansion arm to the expansion side is limited so that the supply amount of pressure oil to the clamshell bucket can be increased as much. Consequently, the clamshell bucket can be opened rapidly, and the subject that the speed at which the clamshell bucket is opened is low can be solved and improvement of the operability can be achieved.
Preferably, the pressure reduction means includes first pressure reduction means for reducing the operating pressure for opening the clamshell bucket and outputting the reduced operating pressure, and second pressure reduction means for reducing the operating pressure for driving the expansion arm to the expansion side based on the output pressure from the first pressure reduction means.
By the configuration, similar advantages to those described above can be achieved. Further, there is an another advantage that the present apparatus can be provided readily at a low cost.
Preferably, the pressure reduction means includes operating pressure detection means for detecting the operating pressure for opening the clamshell bucket, and third pressure reduction means for reducing the operating pressure for driving the expansion arm to the expansion side based on detection information from the operating pressure detection means.
By the configuration, similar advantages to those described above can be achieved. Further, since it is necessary to add only one pressure reduction means as a hydraulic apparatus to a common hydraulic circuit, there is an another advantage that the present apparatus can be provided readily at a low cost similarly.
It is to be noted that, in this instance, further preferably the third pressure reduction means is set so that, as the operating pressure detected by the operating pressure detection means increases, the operating pressure for driving the expansion arm to the expansion side is reduced as much.
According to another aspect of the present invention, a hydraulic circuit for a working machine which includes an expansion arm and a clamshell bucket attached to a tip end of the expansion arm is characterized in that it comprises a regeneration valve interposed between a working cylinder of the expansion arm and an output pressure supply path of the opening side of the clamshell bucket, when the expansion arm is driven to the expansion side, and capable of supplying returning pressure oil from the working cylinder to the output pressure supply path, and a directional control valve for being changed over, in response to an operating pressure for driving the expansion arm to the expansion side, so that an opening operating pressure for opening the clamshell bucket is supplied as a driving operating pressure for the regeneration valve to the regeneration valve to change over a working condition of the regeneration valve.
Accordingly, there is an advantage that, when the operation for expanding the expansion arm and the operation for opening the clamshell bucket are performed in an interlocking relationship, the opening speed of the clamshell bucket can be increased without decreasing the expansion speed of the expansion arm. Consequently, there is another advantage that the subject, that the opening speed of the clamshell bucket is low, can be solved and improvement in operability can be achieved.
Preferably, the directional control valve has a non-response zone within which the driving operating pressure is not supplied to the regeneration valve within a region within which the operating pressure for driving the expansion arm to the expansion side is lower than a predetermined pressure. By such configuration, the expansion arm can be prevented from being expanded suddenly.
Further preferably, the directional control valve is set such that, in another region wherein the operating pressure for driving the expansion arm is higher than the predetermined pressure, the driving operating pressure to be supplied to the regeneration valve increases in response to an increase of the operating pressure for driving the expansion arm to the expansion side. By such configuration, as the operating pressure for expanding the expansion arm increases, the clamshell bucket can be opened at a higher speed.
The regeneration valve may be configured such that, as the driving operating pressure supplied from the directional control valve increases, the amount of returning pressure oil to be supplied from the working cylinder to the output pressure supply path increases. Where the regeneration valve is configured in this manner, sudden expansion of the expansion arm within the region, wherein the driving operating pressure is low, is prevented, and in the region wherein the driving operating pressure is high, the clamshell bucket can be opened rapidly.
According to a further aspect of the present invention, a hydraulic circuit for a working machine which includes an expansion arm and a clamshell bucket attached to a tip end of the expansion arm is characterized in that it comprises a regeneration valve interposed between a working cylinder of the expansion arm and an output pressure supply path of the opening side of the clamshell bucket, and capable of supplying returning pressure oil from the working cylinder when the expansion arm is driven to the expansion side to the output pressure supply path, and that the working condition of the regeneration valve is controlled based on an operating pressure for driving the expansion arm to the expansion side.
Accordingly, by controlling the working condition of the regeneration valve based on the operating pressure for driving the expansion arm to the expansion side, there is an advantage that, when the operation for expanding the expansion arm and the operation for opening the clamshell bucket are performed in an interlocking relationship, the opening speed of the clamshell bucket can be increased without decreasing the expansion speed of the expansion arm.
Consequently, there is another advantage that the subject that the opening speed of the clamshell bucket is low can be solved and improvement in operability can be achieved.